Transcript Hearing

Hearing
Sound is created by vibrations from a source
and is transmitted through a media (such as
the atmosphere) to the ear.
Sound has two main attributes:
1.Frequency of Sound Waves:
•When sound is generated, it causes vibration
and makes the air molecules to be moved
back and forth. This alternation creates
corresponding increase and decreases in the
air
pressure.
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•The vibration forms sinusoidal (sine) waves.
The height of the wave above and below the
midline represents the amount of abovenormal and below-normal air pressure
respectively.
• The waveform above the midline is the
image of the waveform below the midline in a
sine wave. The waveform repeats itself again
and again in a sine wave.
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•The number of cycles per second is called
the frequency of the sound. Frequency is
expressed in hertz (Hz) and is equivalent to
cycles per second.
•The human ear is sensitive to frequencies
in the range of 20 to 20,000 Hz (highest
sensitivity between 1000 to 3000 Hz), but it
is not equally sensitive to all frequencies. In
addition, people differ in their relative
sensitivities to various frequencies.
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2.Sound intensity:
It is defined in terms of power per unit area. The
Bel (B) is the basic unit for measuring sound. The
most convenient measure is the decibel (dB), 1 dB=0.1B.
(see figure 6.2)
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Complex Sounds:
Very few sounds are pure. Most complex
sounds are non-harmonic.
•Masking:
It is the condition when one component of
the sound environment reduces the
sensitivity of the ear to another component.
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AUDIOTORY DISPLAYS
There are four types of human functions involved in the
reception of auditory signals:
1. Detection
2. Relative discrimination (differentiating between two or
more signals presented together)
3. Absolute identification (only one signal is present)
4. Localization (knowing the direction that the signal is
coming from)
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Detection of signals
 Signals can occur in peaceful surroundings or noisy surroundings.
 The signal plus the noise (SN) should be distinct from the noise (N)
itself.
 If the above is not the case, the signal can not be detected in the
presence of noise. The threshold of the detectability of the signal is
elevated.
 This threshold should be exceeded by the signal if it is to be
detected accurately.
 What Level? Annoyance?
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Relative Discrimination
 Relative discrimination of signals on the basis of
intensity and frequency depends in part on
interactions between these two dimensions.
 A common measure is the just-noticeable
difference (JND).
 JND is the smallest difference or change along a
stimulus dimension that can just be detected
50% of the time by people.
 The smaller the JND, the easier it is for people
to detect differences on the dimension being
changed.
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Absolute Identification
 The number of levels along a continuum
(range or scale) that can identified usually
is quiet small.
See table 6-1 (page 174)
 It is better to use more dimensions with
fewer steps or levels of each dimension
than to use fewer dimensions and more
levels of each.
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Localization
 The ability to localize (guess the direction
of sound) the direction from which the
sound is coming from is called
stereophony.
 People mostly relay on intensity and
phase of sound to determine the direction
of sound.
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NOISE
 Noise is referred to as unwanted sound.
 In the context of information theory, noise
is defined as “ that auditory stimulus of
stimuli
bearing
no
informational
relationship to the presence or completion
of the immediate task”.
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How loud is it?
 Loudness depends on intensity and frequency.
 A low frequency tone will not sound as loud as a high
frequency sound of the same intensity.
 One of the most important effects of noise is hearing
loss.
 Occupational hearing loss.
 Temporary loss, permanent loss.
 Continuous noise or noncontinuous noise.
 Physiological effects of noise.
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• TWA - Time Weighted Average Noise Levels - and
Noise Dose
• The TWA shows a worker's daily exposure to occupational
noise (normalised to an 8 hour day), taking into account
the average levels of noise and the time spent in each
area.
• This is the parameter that is used by the OSHA
Regulations and is essential in assessing a workers
exposure and what action should be taken.
• Working Out the Noise Dose and TWA
• Before working out the worker's TWA you have to
measure the different high noise levels that the worker is
subjected throughout a normal working day. The Time
Weighted Average is calculated using these noise levels
together with the amount of time that the worker is
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exposed to them.
First calculate the Noise Dose as:
Time Actually Spent at Sound Level
Partial Noise Dose 
Maximum Permissibl e Time at Sound Level
Total Dose = 100 x (C1/T1 + C2/T2 + C3/T3 + ... + Cn/Tn)
where
Cn = time spent at each noise level
Tn= Permissible time (from table)
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Permissible Noise Exposures According to
OSHA
Sound level dBA
Permissible time, h
80
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85
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90
8
95
4
100
2
105
1
110
0.5
115
0.25
120
0.125
125
0.063
130
0.031
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TWA= [16.61 X log(D/100)] + 90
The link below contains a noise calculator:
http://www.noisemeters.com/apps/occ/twa-dose.asp
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